k8s Ingress: fix crash on rules with nil http

[grimmy]

What does this PR do?

Fixes a crash when using the KubernetesIngress Provider if an Ingress has a rule without a nil http element.

This is a common case if you're using Let's Encrypt http01 verification to create a certificate for a non-HTTP based service. Or in my case, the ingress just uses an annotation to HTTP redirect with ingress-nginx (which i'm migrating away from).

Basically, just adds a nil check. Without this nil check a single ingress rule that has a nil http element will make the entire provider not work.

I also added a unit test to verify the change and to avoid it from happening again in the future.

Motivation

I'm trying to migrate to traefik from ingress-nginx and after hours of searching for why my secrets weren't being found, I finally dug into the nil pointer deference I was also seeing and it turned out that it was the culprit all along.

More

• Added/updated tests
• Added/updated documentation

Additional Notes

This is targeting the master branch as during my testing it appeared that 0286836 may have had some impact on the secret resolution, but ultimately I don't think that really matters

[ldez]

Thanks 👍

[SantoDE]

LGTM. Thanks! :) 👼

[dtomcej]

LGTM